The effect of hydrogen on the crystallization of hydrogenated amorphous silicon (a-Si:H) films has been studied. a-Si:H films were prepared by plasma-enhanced chemical vapor deposition (PECVD) at various hydrogen dilution ratios (H-2:SiH4) and annealed for the crystallization at 873 K in a vacuum furnace. The crystallized grain size and the structural disorder were analyzed by transmission electron microscopy (TEM) and Raman scattering, respectively. The hydrogen-silicon bonding nature and the content of the remaining hydrogen were measured by Fourier transform infrared (FTIR) spectroscopy and hydrogen evolution measurement. In the annealing of the a-Si:H film deposited at higher dilution ratio (12:1) with mainly SiH bondings, the structural disorder drastically increased shortly after the hydrogen evolution, then gradually increased for 5 h due to the relaxation and then decreased until the nucleation started. The large grain size of the fury crystallized film was attributed to the increased structural disorder during annealing. During annealing, the gradual increase of the structural disorder had not been observed for the sample deposited at lower dilution ratio (2:1) with a mixture of SiH2 and SiH bonding. It was found that hydrogen did not influence the nucleation of a-Si:H film directly because most of the hydrogen was evolved in 10 min at 873 K, which was much shorter than the incubation time of 15 h. The grain size of the fully crystallized film has been affected by the change in structural disorder during annealing, which was strongly correlated to the silicon-hydrogen bonding characteristics of the deposited a-Si:H film.